This invention relates to production of polyester fibers of linear glycol terephthalate polymers, and is more particularly concerned with a process for preparing modified fibers of good strength and subsequently developing predetermined pilling resistance properties after the fibers have been spun into yarn and knitted into fabric.
Polyester staple fibers have caused pilling problems in fabrics. Pilling refers to the accumulation on a fabric surface of numerous, unsightly, small balls of entangled fibers. The unsightly effect of pilling is not due so much to the formation of pills, which occurs on fabrics made from any staple fibers, but to the difficulty in wearing off the pills after they have formed. Thus, while high strength and abrasion resistance are desirable in polyester fabrics made from the continuous filaments, they are undesirable in staple fibers because they prevent rapid removal of pills during normal use of the fabrics. The problem is particularly severe in fabric knitted from spun staple yarns.
The pilling problem can be overcome by producing weak fibers. This is accomplished by lowering the relative viscosity. In general, polyester fibers exhibit little or no pilling when the fibers are about 8 to 12.5 relative viscosity (RV.sub.H as subsequently defined). Fibers of less than about 8 RV.sub.H are undesirably weak for most textile uses.
Production of polyester fibers becomes increasingly difficult as the relative viscosity is reduced. The melt viscosity of the polyester becomes so low that maintenance of product uniformity and continuity of spinning of the molten filaments without the formation of "drips" becomes a serious problem. A solution to this problem is to modify the polyester with an oxysilicon compound to increase the molecular weight by forming temporary links between ends of polymer chains. Exposure of the spun fibers to moisture, even the normal exposure of fibers to atmospheric humidity which occurs in fiber-manufacturing operations, causes hydrolysis of the oxysilicon linkages with resultant reduction of the molecular weight to give fibers of low relative viscosity.
Production of fibers having low relative viscosity gives the fiber producer a control over the pilling resistance that provides for improved uniformity of performance in the final fabric but sacrifices strength and abrasion resistance which are desirable during processing of staple into yarn and then into fabric. Weakness of the fibers during processing is a serious problem in producing knit fabrics. Fabrics knit from polyester staple, when compared with woven fabrics, have been found to have a magnified pilling problem because of the looseness of the knit structure, and prior art solutions to the pilling problem have not been fully satisfactory. Thus fibers eminently suitable for worsted fabrics are not at all suitable for knit fabrics. Knit fabrics require polyester staple fibers in a very low molecular weight range if they are to exhibit significant pilling resistance, and polyesters spun with molecular weights in this low range have been found to give weak fibers with accompanying operability problems in preparing and handling fine count yarns. The present invention overcomes this problem.